In most cases, ball valves are provided with plastic or elastomer seats that are maintained in positive seating engagement with a metal valve ball at all times. These particular materials are preferred for use as ball valve seats because of the sealing capability provided thereby and because they do not tend to scratch or erode the spherical sealing surface of the ball as the ball is rotated during opening and closing movement. Moreover, plastic and elastomeric seat members have a certain degree of resiliency and may be slightly deformed so as to maintain a certain degree of mechanical pressure between the seats and the sealing surface of the valve ball to enhance the sealing capability thereof.
In many cases, it is desirable to utilize metal seat elements because of the high pressure sealing capability thereof. It is well known, however, that metal seats can cause rapid deterioration of the sealing surface of valve balls if the valve ball element is rotated while the valve is under high pressure. The reaction between metal sealing surfaces of seat assemblies and the spherical sealing surface of a metal valve ball can cause scratching, erosion or galling of either of these metal sealing surfaces. When this occurs, of course, the valve mechanism will be incapable of developing a positive seal and leakage will occur. It is desirable, therefore, to provide metal sealing surfaces within the valve mechanism to enhance high pressure sealing capability and it is also desirable to provide means for ensuring against damage to the sealing surfaces as the valve mechanism is operated under high pressure conditions.